The progressive phase of multiple sclerosis (MS) is the one most often associated with irreversible accumulation of disability. An important question remains about the place of primary progressive MS (PP-MS): does it form an integral part of the disease spectrum, or is it maybe a distinct entity? This question could apparently be very theoretical, but it is not, as patients with PP-MS remain orphans when regarding disease-modifying treatments. Thus, they are usually excluded from therapeutic trials. A clue to this question could be the comparison between the different MS subtypes with a progressive phase. We discuss here the clinical similarities and differences between secondary and primary progressive MS.